Irvington Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Irvington Observatory clear sky chart from 2005-11-05 to 2017-10-06.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
37%
15984
15%
47201
10%
15942
42%
15942
20%
33032

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
17%
900
9%
2506
3%
895
21%
895
11%
2008
January16th to endNight
20%
882
8%
2610
3%
882
25%
882
11%
2109
February1st to 15thNight
17%
701
9%
2247
3%
701
23%
701
11%
1734
February16th to endNight
24%
631
13%
1893
6%
631
31%
631
14%
1397
March1st to 15thNight
27%
720
14%
2088
8%
720
33%
720
18%
1512
March16th to endNight
33%
768
18%
2112
10%
768
39%
768
21%
1496
April1st to 15thNight
28%
570
13%
1804
7%
570
34%
570
18%
1250
April16th to endNight
34%
540
19%
1620
9%
540
44%
540
25%
1080
May1st to 15thNight
38%
540
13%
1620
8%
540
44%
540
22%
1127
May16th to endNight
49%
562
15%
1654
11%
561
54%
561
29%
1055
June1st to 15thNight
55%
540
15%
1440
11%
540
58%
540
33%
915
June16th to endNight
56%
540
16%
1440
12%
540
61%
540
34%
915
July1st to 15thNight
64%
510
16%
1360
13%
510
67%
510
39%
813
July16th to endNight
67%
564
20%
1482
16%
531
69%
531
40%
873
August1st to 15thNight
66%
539
21%
1609
17%
536
69%
536
35%
960
August16th to endNight
62%
573
22%
1755
18%
573
66%
573
33%
1033
September1st to 15thNight
59%
657
25%
1917
20%
657
63%
657
35%
1128
September16th to endNight
58%
717
24%
1980
18%
717
62%
717
34%
1209
October1st to 15thNight
47%
684
19%
1952
16%
684
51%
684
26%
1298
October16th to endNight
26%
704
14%
2219
8%
704
35%
704
16%
1583
November1st to 15thNight
34%
700
19%
2276
16%
700
39%
700
21%
1484
November16th to endNight
24%
720
13%
2346
9%
720
30%
720
14%
1731
December1st to 15thNight
18%
762
7%
2562
3%
762
22%
762
9%
2014
December16th to endNight
14%
960
7%
2709
1%
960
19%
960
8%
2308

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
14%8%47%6%35%45%7%33%
47984
Night
15%8%46%5%34446%35%
47377
Cloud CoverDay
14%8%47%6%3445%7%33%
47994
Night
15%8%46%5%34446%35%
47381
TransparencyDay
49%6%11%20%10%1
47679
Night
48%5%12%20%11%1
47201
SeeingDay
16%7%27%23%16%8%
5944
Night
19%8%28%29%12%1
15988

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
18%16%6%11%7%4444417%
1432
16th to endDay
16%18%9%9%8%45%33415%
1613
February1st to 15thDay
14%15%7%10%8%45%45%418%
1612
16th to endDay
14%15%6%12%7%345%45%20%
1580
March1st to 15thDay
16%10%6%8%6%35%346%27%
1860
16th to endDay
18%11%48%5%33436%30%
2112
April1st to 15thDay
20%9%45%5%25%446%30%
2154
16th to endDay
16%7%26%5%24446%38%
2340
May1st to 15thDay
19%7%27%5%35%446%32%
2331
16th to endDay
13%6%35%5%35%46%8%38%
2458
June1st to 15thDay
10%5%25%6%35%5%5%8%41%
2520
16th to endDay
10%425%6%46%6%7%10%34%
2520
July1st to 15thDay
8%5%245%35%5%6%9%45%
2520
16th to endDay
8%425%6%35%6%7%9%39%
2583
August1st to 15thDay
10%5%25%5%46%6%5%7%39%
2340
16th to endDay
11%5%25%5%35%5%6%8%40%
2444
September1st to 15thDay
11%5%25%5%25%5%5%7%41%
2031
16th to endDay
12%5%26%43447%7%41%
1980
October1st to 15thDay
15%7%37%5%23446%39%
1806
16th to endDay
22%9%36%6%24346%31%
1650
November1st to 15thDay
15%8%27%5%24446%37%
1582
16th to endDay
17%12%5%8%6%24446%26%
1609
December1st to 15thDay
16%12%8%11%8%35%446%19%
1395
16th to endDay
20%17%8%11%6%3434416%
1512
January1st to 15thNight
22%13%48%6%44345%21%
2506
16th to endNight
14%12%8%12%7%45%446%19%
2610
February1st to 15thNight
16%15%6%8%6%34335%25%
2245
16th to endNight
15%11%5%9%7%35%35%6%26%
1893
March1st to 15thNight
16%11%46%6%2435%5%32%
2076
16th to endNight
15%7%36%5%35%446%35%
2112
April1st to 15thNight
19%8%35%435%45%7%33%
1804
16th to endNight
17%6%2443345%7%39%
1620
May1st to 15thNight
19%5%26%5%2445%6%35%
1620
16th to endNight
12%5%245%35%348%44%
1652
June1st to 15thNight
11%4245%25%35%8%46%
1440
16th to endNight
9%435%5%245%7%10%43%
1440
July1st to 15thNight
7%31442445%8%53%
1440
16th to endNight
6%42442435%9%51%
1583
August1st to 15thNight
5%315%5%345%6%10%47%
1620
16th to endNight
10%5%24423346%51%
1755
September1st to 15thNight
12%42432345%6%52%
1917
16th to endNight
10%5%15%4235%48%47%
1980
October1st to 15thNight
14%5%36%425%45%6%42%
1952
16th to endNight
21%8%36%5%34345%33%
2219
November1st to 15thNight
15%8%25%434446%38%
2276
16th to endNight
20%12%46%5%245%5%5%28%
2346
December1st to 15thNight
20%13%6%8%6%35%435%22%
2562
16th to endNight
21%16%8%9%6%34445%15%
2709

Percentage Cloud Cover SE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
18%17%5%11%6%44445%16%
1432
16th to endDay
16%19%8%9%8%45%33415%
1613
February1st to 15thDay
14%14%6%10%9%5%5%445%18%
1612
16th to endDay
14%14%6%12%7%245%45%20%
1580
March1st to 15thDay
16%11%5%8%6%3435%6%27%
1871
16th to endDay
19%10%47%6%33345%30%
2111
April1st to 15thDay
20%9%45%6%24446%30%
2154
16th to endDay
16%7%35%5%24347%38%
2340
May1st to 15thDay
19%7%26%5%35%45%5%32%
2331
16th to endDay
13%5%35%5%35%46%8%38%
2458
June1st to 15thDay
10%5%35%5%45%5%5%8%41%
2520
16th to endDay
10%5%25%7%36%6%8%10%34%
2520
July1st to 15thDay
7%5%25%435%5%6%8%44%
2520
16th to endDay
7%425%6%36%5%8%9%39%
2583
August1st to 15thDay
9%5%25%5%46%6%5%8%38%
2340
16th to endDay
11%5%246%35%5%5%8%40%
2444
September1st to 15thDay
11%5%26%5%25%46%8%41%
2031
16th to endDay
12%5%25%33447%7%41%
1980
October1st to 15thDay
15%7%37%5%23446%39%
1806
16th to endDay
22%9%35%6%24446%31%
1650
November1st to 15thDay
15%8%37%5%23446%37%
1582
16th to endDay
17%13%5%9%5%34346%26%
1609
December1st to 15thDay
16%13%8%11%7%34446%19%
1395
16th to endDay
21%17%8%10%5%4333416%
1512
January1st to 15thNight
22%13%48%6%34345%21%
2506
16th to endNight
14%12%7%11%7%45%45%5%20%
2610
February1st to 15thNight
16%15%5%8%6%44345%25%
2238
16th to endNight
15%11%5%8%8%35%446%26%
1893
March1st to 15thNight
16%11%46%5%24445%32%
2088
16th to endNight
15%7%46%5%35%5%46%35%
2112
April1st to 15thNight
19%8%35%435%5%47%32%
1804
16th to endNight
17%6%245%2345%7%39%
1620
May1st to 15thNight
20%5%25%5%245%5%7%35%
1619
16th to endNight
12%4245%35%349%44%
1652
June1st to 15thNight
11%3245%2445%8%46%
1440
16th to endNight
9%435%4235%7%10%43%
1440
July1st to 15thNight
8%3144235%5%8%53%
1440
16th to endNight
6%42442445%10%51%
1583
August1st to 15thNight
5%315%5%35%5%5%10%47%
1620
16th to endNight
10%5%25%41345%6%51%
1755
September1st to 15thNight
11%5%13323446%52%
1917
16th to endNight
10%5%25%42345%7%47%
1980
October1st to 15thNight
15%5%35%42445%7%41%
1952
16th to endNight
21%9%26%5%24445%33%
2219
November1st to 15thNight
15%8%25%5%24446%38%
2276
16th to endNight
21%12%46%5%245%5%5%27%
2346
December1st to 15thNight
20%13%6%8%5%35%43422%
2562
16th to endNight
22%16%7%10%6%34435%15%
2709

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
68%5%11%10%3
1433
16th to endDay
72%5%9%8%4
1613
February1st to 15thDay
67%5%8%12%5%
1614
16th to endDay
64%5%9%14%5%1
1580
March1st to 15thDay
56%6%12%16%7%
1871
16th to endDay
55%410%20%7%1
2112
April1st to 15thDay
54%5%10%19%10%
2154
16th to endDay
46%5%13%16%14%2
2340
May1st to 15thDay
52%5%10%19%9%2
2331
16th to endDay
43%6%12%24%11%1
2462
June1st to 15thDay
39%7%13%24%14%1
2520
16th to endDay
40%10%12%20%15%
2520
July1st to 15thDay
34%7%11%25%18%2
2371
16th to endDay
37%8%14%21%15%2
2422
August1st to 15thDay
40%7%13%24%12%1
2327
16th to endDay
39%7%11%23%15%1
2444
September1st to 15thDay
39%7%12%25%14%1
2031
16th to endDay
38%6%11%29%13%
1980
October1st to 15thDay
45%5%11%23%12%1
1806
16th to endDay
54%415%18%6%
1650
November1st to 15thDay
47%5%11%23%11%1
1582
16th to endDay
58%412%17%5%
1609
December1st to 15thDay
65%412%13%3
1395
16th to endDay
72%411%8%3
1512
January1st to 15thNight
64%411%12%6%
2506
16th to endNight
64%6%12%11%4
2610
February1st to 15thNight
61%5%11%14%6%
2247
16th to endNight
57%413%16%6%
1893
March1st to 15thNight
52%5%11%17%10%1
2088
16th to endNight
49%5%11%21%10%1
2112
April1st to 15thNight
49%5%11%18%13%
1804
16th to endNight
43%5%12%18%16%3
1620
May1st to 15thNight
47%5%11%20%10%3
1620
16th to endNight
38%5%13%23%15%3
1654
June1st to 15thNight
34%8%12%28%15%1
1440
16th to endNight
34%7%15%25%17%
1440
July1st to 15thNight
29%5%10%30%20%2
1360
16th to endNight
29%5%15%25%21%2
1482
August1st to 15thNight
30%7%12%32%14%2
1609
16th to endNight
32%6%11%28%18%3
1755
September1st to 15thNight
32%5%10%29%20%1
1917
16th to endNight
34%413%28%17%
1980
October1st to 15thNight
41%6%11%21%17%1
1952
16th to endNight
52%5%12%20%7%
2219
November1st to 15thNight
44%5%12%22%12%1
2276
16th to endNight
56%5%13%17%6%
2346
December1st to 15thNight
64%5%12%12%5%
2562
16th to endNight
70%5%10%9%3
2709

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
20%17%38%15%7%
90
16th to endDay
32%9%38%13%6%
96
February1st to 15thDay
22%22%44%10%1
90
16th to endDay
22%14%38%19%4
81
March1st to 15thDay
28%11%34%19%6%
161
16th to endDay
24%6%34%22%10%1
288
April1st to 15thDay
24%8%32%24%7%2
270
16th to endDay
20%12%28%20%11%7%
270
May1st to 15thDay
25%10%26%19%12%5%
288
16th to endDay
16%7%26%23%17%8%
358
June1st to 15thDay
12%6%32%28%15%4
495
16th to endDay
11%7%32%25%17%6%
495
July1st to 15thDay
10%5%24%28%21%10%
475
16th to endDay
8%421%31%23%11%
443
August1st to 15thDay
12%115%28%19%23%
313
16th to endDay
16%315%20%25%18%
332
September1st to 15thDay
11%5%18%19%26%19%
315
16th to endDay
15%216%20%28%15%
314
October1st to 15thDay
15%6%24%28%17%6%
278
16th to endDay
29%8%30%24%7%
162
November1st to 15thDay
10%17%24%31%14%2
90
16th to endDay
23%17%37%17%4
88
December1st to 15thDay
21%20%35%20%3
60
16th to endDay
28%23%41%6%
92
January1st to 15thNight
26%13%35%19%4
900
16th to endNight
20%11%35%27%5%
882
February1st to 15thNight
24%12%36%22%3
704
16th to endNight
20%12%32%28%4
631
March1st to 15thNight
23%11%31%26%6%
720
16th to endNight
18%10%32%28%8%1
768
April1st to 15thNight
23%11%33%24%5%
570
16th to endNight
20%6%35%26%9%1
540
May1st to 15thNight
22%5%28%33%9%1
540
16th to endNight
14%325%36%17%2
563
June1st to 15thNight
12%223%45%16%
540
16th to endNight
10%221%45%19%
540
July1st to 15thNight
10%116%38%28%2
510
16th to endNight
6%217%47%24%2
564
August1st to 15thNight
8%16%43%26%3
539
16th to endNight
12%215%33%30%5%
573
September1st to 15thNight
13%318%31%27%5%
657
16th to endNight
12%417%30%29%5%
717
October1st to 15thNight
17%5%25%32%16%2
684
16th to endNight
26%10%31%24%5%
704
November1st to 15thNight
19%10%30%25%12%1
700
16th to endNight
26%11%31%22%7%
720
December1st to 15thNight
25%13%36%20%3
762
16th to endNight
30%11%34%20%3
960
Page generated at 2017-10-07 14:07:47. Version ac04

Copyright 2017 A. Danko


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